One of the main problems confronting the semiconductor processing industry, in the ULSI age, is that of capacitive-resistance loss in the wiring levels. This has led to a large effort to reduce the resistance of and lower the capacitive loading on wiring levels. Since its beginning, the industry has relied on aluminum and aluminum alloys for wiring. In a like manner, the industry has mainly relied on SiO2 as the insulator of choice, although polyimide was used in a number of products by IBM, for a number of years. The capacitive resistance problem grows with each succeeding generation of technology.
To improve the conductivity, it has been suggested to substitute copper, silver or gold metallurgy for the aluminum metallurgy now being used. Several potential problems have been encountered in the development of these proposed metallurgies. One problem is the fast diffusion of copper through both silicon and SiO2. This problem, along with the known junction poising effects of copper and gold, have led to proposals to use a liner to separate these metallurgies from an SiO2 insulator. These approaches, however, do not fully resolve problems associated with decreasing minimum line size and decreasing liner size. The combination of the shrinking line size in the metal line and the decreasing liner size increases both the capacitance and resistance.
With respect to capacitive loading effects, studies have been conducted that considered employing various polymers such as fluorinated polyimides as possible substitutions for SiO2 insulators. Several of these materials have dielectric constants considerably lower than SiO2. However, as in the case of SiO2, an incompatibility problem with copper metallurgy has been found. In the case of polyimide, and many other polymers, it has been found that the polymer, during curing, reacts with copper during the curing process, forming a conductive oxide CuO2, which is dispersed within the polymer. See, D. J. Godbey et al., International Conference on Metallurgical Coatings and Thin Films, San Diego, Calif. Abstract H2.04, pg. 313 (Apr. 21-25, 1997). This conductive oxide then raises the effective dielectric constant of the polymer and in many cases increases the polymers conductivity.
Various approaches using copper and other metals in SiO2 and polymer insulators have been implemented to improve the properties of the electrical interconnects. However, these electrical characteristics are impaired when an oxide is formed at or near the surface of the electrical interconnect. Thus, structures and methods are needed which alleviate the problems associated with via and metal line fabrication processes.